<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <script src="https://cdn.jsdelivr.net/npm/vue@2.6.13"></script>
</head>
<body>
  <div id="app">
    <div>父组件: {{ num }}</div>
    <my-component v-model="num" />
  </div>

  <script>

    Vue.component('my-component', {
      props: {
        value: Number
      },
      watch: {
        value() {
          this.$emit('input', this.value)
        }
      },
      template: `
      <div>
        子组件：{{ value }} <br/>
        <select v-model="value">
          <option value="1" :selected="true">选项1</option>
          <option value="2">选项2</option>
          <option value="3">选项3</option>
        </select>
      </div>
      `
    })

    const app = new Vue({
      el: '#app',
      data() {
        return {
          num: 1
        }
      }
    })
  </script>
</body>
</html>